This invention relates to powder coating compositions and to their use.
Powder coating compositions generally comprise a solid film-forming resin, usually with one or more colouring agents such as pigments, and optionally they also contain one or more performance additives. They are usually thermosetting, incorporating, for example, a film-forming polymer and a corresponding curing agent (which may itself be another film-forming polymer).
The compositions are generally prepared by intimately mixing the ingredients (including any colouring agents and performance additives) for example in an extruder, at a temperature above the softening point of the film-forming polymer(s) but below a temperature at which significant pre-reaction would occur. The extrudate is usually rolled into a flat sheet and comminuted, for example by grinding to the desired particle sizes (xe2x80x9cmicronisingxe2x80x9d). The size distribution required for most commercial electrostatic spray apparatus is up to 120 microns, often between 10 and 120 microns, with a Dv.50 within the range of 15 to 75 microns, preferably 25 to 50 microns, or more especially 20 to 45 microns.
Currently, powder coating manufacturing methods allow the manufacturer to offer commercially a range of full gloss coatings in a variety of colours. The range of products available in reduced-gloss finishes is, however, limited.
Reduction of gloss to some other lower level, for example satin gloss (55-65% gloss) or matt ( less than 30% gloss), is achieved by creating a surface which is rough on a microscopic scale. This may be achieved by using incompatible components or components that generate incompatibility. For example, an acrylic component and a polyester, epoxy, polyester-epoxy or polyurethane component are incompatible, and cannot be blended to form a single (stable) phase. Incompatibility during film formation can also be achieved by using components that are initially miscible (compatible) but that become incompatible during curing. Thus, for example, two systems of similar chemistry and approximately the same gel time are compatible, but components with different gel times are initially compatible but become incompatible as curing (and molecular weight build-up) proceeds. Thus, gloss-reducing agents include a second catalyst which will give a much faster gel time than the principal catalyst used to cure the film. Alternatively, with acid-functional polyesters, for example, a fast gelling (reacting) powder and a slow gelling powder may be manufactured separately using polyesters of different functionality, and mixed after the micronising stage or, more usually, the components are mixed prior to micronising; the components should have the same colour and particle size. Production of small batches of reduced-gloss coating composition, however, is uneconomic. A different method utilises a product called xe2x80x98gloss killerxe2x80x99, available from the company Tiger, which is mixed in after the micronising stage. However, this product, a clear (uncoloured) powder coating, can be added only in limited amounts to a conventional coloured powder coating before the presence of the gloss killer can be detected from the sparkle generated by the particles of the dear gloss killer powder in the film. Therefore the product is limited to adjusting gloss by a few percentage points.
There is accordingly a need for powder coating compositions in a wide range of reduced-gloss finishes, which avoid the problems mentioned above.
Speed of delivery to customers is also an important consideration, but the conventional powder coatings manufacturing method relies on premixing, extrusion and milling as separate processes, causing turnaround times to be long, and production of small batches of a product is again not economic. Stocking large product ranges solves the problem of speed of delivery, but is a highly inflexible approach and is not cost-effective.
EP 372860 A describes a colour mixing process for powder coatings in which sufficiently small-sized particles ( less than 20 xcexcm in size and advantageously  less than 10 xcexcm) are used that mixed colours applied to a substrate have a homogeneous appearance. Before application to the substrate the mixture is generally subjected to a process of agglomeration in which the small-sized particles are fused or bonded into composite particles, for example by mechanofusion, to convert the mixture from a cohesive mass to a free-flowing and fluidisable powder, which can be applied by conventional means. Thus, a range of basic coloured powder coating compositions is produced, conventionally, in a conventional melt extrusion step, and the products are comminuted to a distinctively small particle size. A range of other colours can then be produced by mixing and agglomerating these coloured powder bases in the desired proportions. This allows the storage of comparatively few basic coloured powder bases, which can be mixed and agglomerated to produce any desired shade easily on demand, and the production of small quantities becomes commercially feasible.
An extension of this agglomeration technique for the flexible production of a range of coloured powder coatings with a range of different reduced-gloss and other aesthetic effects is described in EP 539385 A.
Gloss reduction is achieved, for example, by adding uncoloured incompatible particles of mean particle size  less than 5 xcexcm, or initially compatible particles of mean particle size  less than 20 xcexcm of a polymer having a different functionality from the polymer of the main film-forming component. Amounts of the gloss-reducing agent are, for example, 5 or 10% by weight, although amounts up to 40% by weight are also disclosed.
The present invention provides a powder coating composition in which powder particles are an agglomerate of individual particulate components fused or bonded together into composite particles, wherein the individual particulate components comprise
(1) one or more coloured film-forming base components having a Dv.99 of no more than 30 xcexcm, and
(2) one or more uncoloured film-forming components, wherein at least one component (2a)
(i) has a Dv.99 that is more than 20 xcexcm and
(ii) has a higher Dv.99 or higher Dv.50 than component (1) or, when there is more than one such component, than those components taken together, the ratio of coloured film-forming base component(s) to uncoloured film-forming component(s) in the composition being in the range of from 1:99 to 60:40 by weight.
As will be understood in the art, the volume percentiles Dv.x indicate for a stated particle size (D) the percentage (x) of the total volume of the particles that lies below the stated particle size; the percentage (100xe2x88x92x) of the total volume lies at or above the stated size. Thus, for instance, Dv.50 is the median particle size of the sample, and on a particle size distribution graph Dv.99 is the point on the curve read along the particle size axis where the area under the curve below this particle size represents 99% by volume of the particles. Thus, Dv.99=30 xcexcm indicates that 99% of the particles are below 30 xcexcm (but are not below 29 xcexcm). (For the avoidance of doubt, it should be noted that all particle sizes quoted herein are by volume.) Volume percentiles are measurable by laser diffraction techniques, for example by the Malvern Mastersizer.
Surprisingly, even with a high proportion of uncoloured component of significantly larger particle size than the coloured base components (whether judged according to Dv.99 or according to Dv.50), nevertheless the particles of the larger-sized component are not visible to the naked eye, so that the composition gives the appearance of a single colour.
A coloured base component comprises colouring material and film-forming polymer. Thus, for example, it may be constituted as a powder coating composition in its own right, containing the usual powder coating additives, but having the specified (reduced) particle size. Usually there will be at least two coloured base components in the composition, each having a Dv.99 of no more than 30 xcexcm.
An uncoloured film-forming component is usually also a powder coating composition in its own right, but having the specified particle size; there should be at least one uncoloured film-forming component having a higher Dv.99 and/or a higher Dv.50 than a coloured film-forming base component. (Usually, a component having a higher Dv.99 than another will also have a higher Dv.50 and vice-versa) Usually, there are two or more coloured film-forming base components each having a Dv.99 of no more than 30 xcexcm and at least one uncoloured film-forming component having a Dv.99 that is more than 20 xcexcm and that is higher than the Dv.99 of the coloured film-forming base components taken together (or having a Dv.99 greater than 20 xcexcm and a Dv.50 that is higher than the Dv.50 of the coloured film-forming base components taken together). Very often that uncoloured film-forming component will have a higher Dv.99 (or a higher Dv.50) than each coloured film-forming base component. Preferably the uncoloured film-forming component has a Dv.99 of at least 40 xcexcm, for example at least 50 xcexcm, or more than 55 xcexcm, e.g. in the range of from 50 to 65 xcexcm. Preferably it will have a Dv.90 of no more than 75 xcexcm and advantageously will have a Dv.50 of at least 8 xcexcm, especially in the range of from 12 to 30 xcexcm. Such uncoloured film-forming components have the advantage of lower manufacturing cost in relation to the small-sized coloured components and their use allows easy adjustment of the polymer/pigment ratio in the composition.
The present invention also provides a kit comprising the following separate components for agglomerating into powder coating compositions for the preparation of powder coatings in a number of different finishes:
a plurality of coloured film-forming base components, each with a Dv.99 of no more than 30 xcexcm, and
one or more uncoloured film-forming components, at least one having a higher Dv.99 or Dv.50 than each of the coloured film-forming components, and a Dv.99 that is more than 20 xcexcm, preferably more than 40 xcexcm and preferably no more than 90 xcexcm.
Such kits allow a rapid and flexible manufacture of a range of coloured powder coating compositions, reduced-gloss finishes being manufactured by the use of an uncoloured component that is incompatible with the coloured base component or that becomes incompatible therewith during film-formation.
The present invention also provides a powder coating composition comprising a non-agglomerated mixture of the individual particulate components specified above in the specified proportions.